The present invention relates broadly to a laser radar apparatus, and in particular to a pulsed radar apparatus utilizing laser operation.
With the development of laser devices providing good frequency stability, optical systems for Doppler homodyne detection have become possible. In a typical laser Doppler system, the laser beam is expanded in diameter and is directed at a target. The radiation back reflected and scattered by the target is received by receiver optics and directed to a detector. A portion of the laser beam is split off prior to transmission and forms the local oscillator beam. This local oscillator beam is also directed to the detector to produce Doppler signal.
One form of laser Doppler system is a co-linear off-axis system. In this system, the laser beam is split by a beam splitter into the output and local oscillator beams. The output beam is directed to an off-axis Cassegrain telescope which provides beam expansion. The returning beam is reduced by the same Cassegrain telescope and is directed to a detector. The local oscillator beam is combined with the returning beam by means of a beam splitter. A disadvantage of the co-linear transmitter-receiver system is the inherent 6 db signal loss. This signal loss is due to the 50% beam splitter which is shared by the output and returning beams.
On-axis systems (systems in which the beam expander optics are located on-axis rather than off-axis) have additional disadvantages. These on-axis systems, which typically use a Cassegrain telescope, can reflect a large amount of laser energy under variable phase and frequency back into the laser and thus broaden the frequency spectrum of the laser. The performance of the system, therefore, is degraded.
One system which overcomes the 6 db signal loss problem uses a plane mirror having a hole through it. Both sides of the plane mirror are reflecting surfaces. The laser beam is expanded by beam expanding optics and the expanded beam is directed to the plane mirror, which is oriented at 45.degree. to the direction of propagation of the expanded beam. Most of the beam is reflected by the first surface of the mirror and is directed at a cube corner reflector. The cube corner reflector reflects the beam to the target. A small portion of the beam, however, passes through the hole to become the local oscillator beam. The returning beam from the target is reflected off the second surface of the mirror and directed along a common path with the local oscillator beam. The returning beam and the local oscillator beam pass through beam reducing lenses which focus the returning beam and the local oscillator beam on to the detector.
The disadvantages of this system are that it requires on-axis beam expanding optics. The on-axis arrangement results in back reflection which can degrade laser performance. In addition, the system uses several lenses to expand the output beam and to reduce te returning beam. When an infrared laser such as a CO.sub.2 laser is used, these lenses can be very expensive.